JPH03253128A - Optical heterodyne cp-fsk system transmitter/receiver - Google Patents

Abstract

PURPOSE:To determine the center frequency of an IF and the value of a modulation exponent and to attain optimum optical communication by setting up the center frequency of an intermediate frequency signal to an integer value 3 corresponding to a half of the transmission speed of a signal to be transmitted. CONSTITUTION:An optical signal inputted to a receiver 5 through an optical fiber 4 is applied to a photocoupler 7, a signal applied from a local oscillation laser diode 6 and the received signal are combined (added) and the combined signal is converted into an intermediate frequency band by a front end circuit 8. A delay detecting circuit 12 delays a half of inputted signal power for a prescribed time and a mixer multiplies the delayed signal by the non-delayed signal and demodulates the multiplied signal. In this case, the center frequency of the intermediate frequency signal is set up to an integer (N)>=3 corresponding to a half of the transmission speed of the signal to be transferred. Consequently the IF center frequency and the modulation exponent value can be determined and the optimum optical communication can be attained.

Description

[Detailed Description of the Invention] [Summary] Regarding an optical heterodyne CP-FSK transmitting/receiving device that performs FM modulation on light using a digital signal and transmits the digital signal, the values of the LF center frequency and modulation index are determined to achieve optimal optical communication. The purpose of the present invention is to provide an optical heterodyne CP-FSK transmitter/receiver that performs the following: In the optical heterodyne CP-FSK method, the center frequency of an intermediate frequency signal is reduced to 3, which is half the transmission speed of the signal to be transferred.
It is configured to be an integer multiple of the above.

[Industrial application field]

The present invention relates to an optical communication system, and more particularly to an optical heterodyne CP-FSK transmitting/receiving apparatus that performs FM modulation on light using a digital signal and transmits the digital signal.

[Conventional technology]

Coherent optical transmission systems are expected to be a next-generation long-distance, high-capacity communication system because they provide high sensitivity characteristics and enable optical frequency multiplexing transmission. Among them, the optical heterogain CP-FSK method (phase continuous = FSK method) is the method closest to practical use because it enables high-speed direct modulation of a laser diode (LD).

In the above-mentioned transmission, the current injected (flowed) into the laser diode is modulated on the transmitting side to give a difference ΔF (ΔF = mBm = modulation index, B: transmission speed) between the optical frequencies corresponding to "1" and "0". and outputs it as an optical CPFSK signal.

The light is then transmitted through optical fiber. On the receiving side, this optical signal is converted into an intermediate frequency band by optical heterodyne detection, and the intermediate frequency signal is demodulated by a demodulator to obtain a baseband signal.

Such a system enables high-speed long-distance transmission using light, making it possible, for example, to create a communication system with low cost per channel.

[Problem to be solved by the invention]

When installing an optical communication system using the optical heterodyne CP-FSK method described above, it is necessary that the transmitter and receiver can be adjusted and assembled independently. That is, the transmitter and receiver are adjusted separately, for example, Company A manufactures the transmitter and Company B manufactures the receiver.

The difference ΔF between the frequencies corresponding to 1 and 0 mentioned above is the modulation index m
The demodulator must be made to obtain the maximum amplitude corresponding to this modulation index. However, if the transmitter and receiver adjust the value of m differently,
The problem was that communication was not possible. That is, in the past, the transmitter and receiver were adjusted as one unit, and there was a problem in that, for example, it was not possible to replace the device between different companies.

Furthermore, the error rate and the like in the entire system vary depending on the modulation index, and a highly efficient optical communication system is required.

SUMMARY OF THE INVENTION An object of the present invention is to provide an optical heterogain CP-FSK transmitting/receiving device that determines the values of an IF center frequency and a modulation index and performs optimal optical communication.

[Means and effects for solving the problem] In the optical heterodyne CP-FSK system, the center frequency of the intermediate frequency signal is set to a half-integer multiple of the transmission speed (intermediate frequency signal f+y=BXN
/2 B: transmission speed, N: positive integer of 3 or more). In the demodulator, if the IF signal input to the multiplexer leaks and enters the baseband band, it will become noise, so the intermediate frequency flF should not be higher than the sum of half the intermediate frequency band and the baseband band. must not. Therefore, by setting N to a positive integer of 3 or more, this leakage is prevented.

In addition, the difference in optical frequencies corresponding to marks and spaces may be set to N/(2n+1) times the transmission speed (n is a positive integer that satisfies 2n+1≦2N) or N/2n times (n satisfies n≦N). By setting the value to a positive integer, it is also possible to take correspondence depending on the type of delay detector.

〔Example〕

The present invention will be described in detail below using the drawings.

FIG. 1 shows the constituent countries of an embodiment of the present invention. The transmitter 1 includes a laser diode (LD) 2 and a drive circuit 3 that drives the laser diode.

A baseband signal St (signal to be transmitted) is added to the drive circuit 3, and the drive circuit 3 controls the current flowing through the laser diode 2 in response to the baseband signal St.

The optical frequency of the laser diode 2 changes depending on the flowing current. The baseband signal Si is a digital signal, and the drive circuit 3 drives it so that the frequency difference ΔF between 1 and 0 becomes ΔF=mB. Here, m is the modulation index, B is the bit rate of the input signal, the input signal is, for example, a signal of B=2.5 Gb/s, and the laser diode is a 1.54 μm 31i-pole DFB-LD module. The optical signal transmitted and modulated by this laser diode is applied to a receiving device 5 via an optical fiber 4.

The optical signal input from the fiber is first applied to the optical coupler 7, where the signal applied from the local laser diode 6 and the above-mentioned received signal are combined (added) and input to the front end circuit 8. This front-end circuit is a balanced receiver and is converted to an intermediate frequency band by this front-end circuit. In other words, the local laser diode 6
and an optical signal manually input via the fiber 4 are mixed, and the frequency component of the difference is output from the front end circuit 8. That signal, that is, the signal in the intermediate frequency band, is I
The F amplifier 9 amplifies and the band pass filter (BPF) 10
In addition to limiting the bandwidth.

The output of the bandpass filter 10 is input to the AFC[1r11], and the current value is controlled by the AFC circuit 11 in accordance with the frequency (average frequency) of the input signal to drive the local laser diode 6.

In other words, by this AFC circuit 11, the difference between the average optical frequency of the optical signal generated from the transmitting side laser diode and the optical frequency of the local laser becomes the center frequency in the bandpass filter 10, and is outputted by the front end circuit 8. It is configured as follows.

The output of the bandpass filter 10 is also applied to a delay detection circuit 12, where half of the input signal power is delayed by a specific time (τ), and a mixer 13 multiplies it with a signal that is not delayed and demodulates it.

FIGS. 2(a) and 2(b) are block diagrams of the demodulation circuit. In Fig. 2 (a), the input signal is 3 dB coupler 20
One is the delay circuit 21 (delay time τ)
and the other directly to the mixer 22, where they are multiplied and demodulated. 2), the signals are divided in a 90° hybrid circuit (3 dB), and similarly, one signal is applied to the delay circuit 21 and the other signal is directly applied to the mixer 22, where they are multiplied and demodulated. This demodulated signal is applied to a low pass filter (LPF) 15, passes only the baseband band, and is applied to an identification circuit 16. This identification circuit converts it into a 0.1 signal and outputs it.

Through the operations described above, the receiving circuit 5 demodulates the received optical signal into a baseband signal.

In the configuration of the embodiment of the present invention described above, the setting of the IF must be determined in consideration of the following points.

■ Bandwidth characteristics of IF amplifiers, optical front-end circuits, mixers used in demodulators, etc., ■ Leakage into the baseband band of the IF band in the coupler, ■ Compatibility with other systems, ■ Line width of the light source, and characteristics of the optical fiber. There are effects of dispersion, etc. −
Microwave components can be used for boat fishing, for example, up to about 2QGHz. However, these are all 1-4GHz
, 2-8 GHz, and 4-16 GHz, which are generally seriesized in multi-octave units. For this purpose, it is desirable that both of these IF multiplied signal basebands fall within these bands. At that time, it is necessary that the IF leakage of the current leak does not fall within the baseband band, as described in the above-mentioned item (2).

When the IF multiplied signal is input to the demodulator 12 and the baseband is output by the demodulator, the IF multiplied signal input to the mixer leaks and enters the baseband band, causing noise and deterioration of sensitivity. Therefore, in order to prevent leakage into the baseband band, f IF > BLPF + B
The condition must satisfy tpr/2.

Here, flF is the center frequency of the IF band E3, and tpr is L
Bandwidth of PF, BIFF is the bandwidth of BPF. By satisfying the upper inequality within a multi-octave unit under the above conditions, it is possible to prevent deterioration of sensitivity in the intermediate frequency band. Namely, as mentioned above.

■It is possible to satisfy item 1.

In addition to this, in order to ensure consistency with other systems as mentioned in item (2) above, for example, N
Set to B/2 (N: positive integer). That is, CP-
Even in the case of the FSK system, by setting the center frequency to NB/2, devices in the IF band can be shared.

Also, as in item (2), it is desirable to reduce the influence of the line width of the light source and the dispersion of the optical fiber. As the modulation index increases, the delay time τ of the delay detection circuit 14 becomes shorter, and the phase noise <Δφ2>=2πΔνπ becomes smaller, and the characteristics tend to be good. However, since the frequency changes are large, the wavelength dispersion of the fiber causes significant deterioration during long distance transmission. In other words, it is undesirable whether the modulation index is large or small. Among the various conditions above, ■～
From the condition of item (2), f rr = 2 B is effective.

In this case, m=4/(2n+1), and considering the condition (2), when n=2, m=0.8 is considered to be optimal. Note that this value of 0.8 is the highest in terms of characteristic sensitivity in the characteristic diagram of the receiving sensitivity for the modulation index shown in FIG. That is, (Receiver 5ensi ti
modulation index (Modulation 1st)
ex), the one with the highest sensitivity is exactly 0.8.

Note that when the bit rate is high, the condition of item (2) becomes especially severe, and it is possible to set f IF = 1.5B and m = 0.6 in terms of design.

The system of the present invention has been explained above, and the selection of the combination of n and N is further explained in 2 in FIGS. 3 and 4.
, a CPFSK method with a bit rate of 5 Gb/S will be further explained. In the demodulation circuit diagram 12 shown in FIG. 2, the explanation will be centered on the 3 dB branch. The IF center frequency may be 1.5B, 2B, 2.5B, etc. based on NB/2. From the point of view of Mokisa's IF leak, for example, BIPF=
When setting it to 2B, B LFF > B / 2, so flF must be 1.5B. That is, IP
Since we want the center frequency to be as low as possible, 1.5B and 2B are selected. That is, in the case of 1.5B, n=3 m<o,s cannot be adopted.In the case of 2B, n=41f<0.5 cannot be adopted.

From the viewpoint of fiber dispersion, if m>1, the degradation during long-distance transmission is large, and if we exclude that a large IF bandwidth is used, m = 0 in the case of 1.5B, m = 0 in the case of 6.2B. 0
．． 8, 0.57 is considered. Also, regarding the line width, m
Since the larger the value, the better, f Iv = 28 and m = 0.8 are considered optimal.

The embodiments of the present invention have been described in detail above.

In the embodiment of the present invention, the case of a bit rate of 2.5 Gb/S has been described, but the present invention is not limited to this, and the same effects can be achieved for other bit rates. In particular, by setting the intermediate frequency to be an integral multiple (N) of half the transmission speed, which is the bit rate, efficient data transmission becomes possible.

〔Effect of the invention〕

As mentioned above, in the optical heterodyne CP-FSK method, efficient conditions are set in the high sensitivity region, so the optical heterodyne CP-FSK method that performs optimal optical communication
An SK type transmitting/receiving device can be obtained. Further, if settings are made to satisfy this condition, communication can be performed optimally even if the receiver and transmitter are individually adjusted.

[Brief explanation of drawings]

Fig. 1 is a block diagram of an embodiment of the present invention, Figs. 2 (a) and (b) are block diagrams of a demodulation circuit, Fig. 3 is a characteristic diagram of receiving sensitivity versus modulation index, and Fig. 4 is a graph of received power and Characteristic diagram of error rate, 1... Transmitting device, 2 ・ 3 ・ 4 ・ 5 ・ 6 ・ 7 ・ 8 ・ 9 ・ 10 ・ 11 ・ 12 ・ 13 ・ 14 ・ 15 ・ 16 ・ LD.・Drive circuit, ・Fiber, ・Receiver, ・Local LD, ・Optical coupler, ・Front end circuit, ・IF amplifier, ・BPF.・AFC.・Delay detection circuit, ・Mixer, ・Delay circuit, ・LPS.・Identification circuit.

Claims (1)

[Claims] 1) In the optical heterodyne CP-FSK system, an optical heterodyne characterized in that the center frequency of the intermediate frequency signal is an integral multiple (N) of 3 or more of half the transmission speed of the signal to be transferred. CP-FSK system transmitting/receiving device. 2) The difference in optical frequencies corresponding to the marks and spaces of the optical signal of the optical heterodyne CP-FSK method is multiplied by N/(2n+1) (n is a positive integer satisfying 2n+1≦2N) times the transmission speed. The optical heterodyne CP-FSK transmitting/receiving device according to claim 1. 3) The difference in optical frequencies corresponding to the marks and spaces of the optical signal of the optical heterodyne CP-FSK method is set to be N/2n (here, n is a positive integer satisfying n≦N) times the transmission speed. An optical heterodyne CP-FSK transmitting and receiving device characterized by: 4) The optical heterodyne CP-FSK transmitting/receiving apparatus according to claim 1, wherein FSK modulation is performed using 0.8 times the transmission rate as a modulation index.